Enceladus: a song of ice and tides

An artist impression of Cassini diving into Enceladus water plumes. credit: NASA/JPL

Cassini will terminate its 20-odd-years-long mission in September. But it’s determined to go out with a bang. In yesterday’s press conference, NASA announced that the probe, during a 2015 flyby of Saturn’s moon Enceladus, found clues that the ocean within the icy moon has almost all we think it needs to spark life.

Enceladus is a fascinating world, with an ice version of Earth’s tectonic activity. Like Earth, Enceladus has volcanoes on its surface, but they spew water, which is what Cassini investigated. Instead of magma, in fact, its surface floats on a gigantic salty ocean. This ocean, NASA announced, seems now the place to go look for life in our solar system.

An illustration of the interior of Enceladus: its icy crust, rocky core and liquid ocean in between. credit: NASA/JPL-Caltech

But that is way off the habitable zone! Shouldn’t Enceladus and all the other ocean worlds be frozen solid all the way through?

They might just escape an icy death using an unusual tool: tides. For astronomers, tides are the difference between gravitational pulls on different sides of a planet or moon. For example, one side of Enceladus is closer to Saturn than the other, so it feels a little more gravity. Since it pulls more on one side than the other, the tidal force stretches Enceladus.

As Enceladus moves around, it gets stretched and pulled in ever-changing ways. So its crust and its interior have to rearrange themselves all the time under this force, parts move and slip on each other. The friction warms the planet up, in a process called tidal heating.

It happens to Earth as well, of course, with tidal forces from the Moon and the Sun. But our planet has an enormous amount of heat left over from its formation, enough to melt rock into magma. Tidal heating doesn’t do much here.

To keep this heat in, Enceladus’ ocean has another unusual ally: the kilometers-thick ice crust over it. Ice is a pretty good thermal insuator, and acts like a giant blanket around the ocean, keeping the freezing void out and precious heat in.

We always focus on what sort of atmosphere planets must have to harbor life, or how far they have to be from cold, dim stars. But it might turn out that a big fat ice cover and powerful tides might go quite some way.

If you want more
  • NASA, as usual, put together a great package with al lot of info on Enceladus and other ocean worlds
  • While sufficient to keep oceans all over some of Jupiter’s moons, tidal heating doesn’t seem to be enough to maintain an ocean around all of Enceladus

Cover photo: CC0 Tilgnerpictures/pixabay

Every snowflake is unique

No Christmas landscape is complete without snow. Lots of snow. And every little snowflake is unique, everyone knows that! How come, tho?

Snow is nothing else than teeny tiny ice crystals that form in the clouds and stay solid all the way down to the ground. Water crystallizes around microscopic imperfections, like dust particles floating in the clouds. Once the initial nucleus is formed, the microscopic droplets gather around it very rapidly.

Even though all snowflakes are somewhat hexagonal (due to the geometry of water molecules), each of them grows in slightly different conditions. Some had more water droplets close by, some other was in a portion of space a fraction of a degree warmer. Each and every factor counts: the final shape of the crystal is sensitive to exactly everything.

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Snowflake Sculpture 3, CC-BY-NC Julie Falk, via Flickr.

So the shape of each snowflake is random, it’s like rolling a die with infinitely many faces. You never know what will come out and all outcomes are different. If you want to put it in more physics-pompous terms, the formation of snowflakes is a stochastic process.

In the end, each snowflake is a picture of the exact conditions in which it formed. And since it’s impossible to reproduce the exact same conditions twice, each of them paints a slightly different picture.

Like pictures, snowflakes too come out better if the scene doesn’t move too much. Indeed, if conditions don’t stay relatively constant around the budding crystals, anything can happen. Most of the times, several crystals aggregate in one big snowflake, sort of a little snowball, which look a lot more like each other.

Regardless of the conditions, it’s really hard to tell them apart anyway.

Happy holidays from amorefisico!

Cover photo: Snow leopards playing in the snow, CC-BY-ND Tambako The Jaguar, via Flickr. Some rights reserved.

Frozen bubbles

In the cold of Canada, a man blows a soap bubble, which immediately freezes. Since that’s awesome, the guy makes a nice video about it.

Who knows, maybe he even knew about all the physics that was going on in front of his camera.

Let’s start from the easiest thing: why does the bottom of the bubble freeze last? The answer is gravity: the outermost layers of the bubble slide down, so the base just has more water to freeze. And it takes longer.

But why does it freeze in spots, instead of just from the top down?
Because cold is not the whole story. Water needs a point to start building its crystals, if it can’t find one, it stays liquid well below zero degrees (Celsius), but is very unstable and freezes at the slightest disturbance.

In a stunning turn of events, a soap bubble has plenty of soap molecules floating around, and they are excellent starting points for the ice crystals we see growing on the surface.

Finally, why does the bubble pop instead of just staying there? In the comments to the video, the author says he popped it, but I do believe he could have just waited a little.

The air he blew in the bubble came from his lungs, so it was over 30 degrees. Pressure inside the bubble, initially, is the same as the atmospheric one, but it drops as the air cools down. This puts a lot of stress on that thin and stiff ice surface. Sooner rather than later it was bound to collapse anyway.

All this magnificent physics in less than 30 seconds. What a wonderful world…

 

Foto: Frozen, CC-BY-NC-ND Benjamin Lehman, via Flickr. Some rights reserved.